1. Field of the Invention
The present invention relates to the heterodyne detection of the optical image of targets in an area illuminated by a laser. The invention more specifically applies to the detection of an image in the infrared range by a mosaic of photodetectors.
2. Description of the Prior Art
It is known that the signal-to-noise ratio (S/N ratio) of infrared detectors is generally limited by the thermal noise and dark current. In the case of such detectors it is not possible in direct detection to reach the value limited by the shot noise for the S/N ratio. However, it is known that this limit ratio can be reached by superimposing on the wave front received from the illuminated objects, a coherent wave of a sufficient intensity and which is supplied by the laser used for illumination purposes.
If the frequency F.sub.L corresponding to this reference wave is the same as the frequency F.sub.S used for the illumination, we have a homodyne detection. If the frequencies F.sub.L and F.sub.S differ, the detection is heterodyne and the electric detection signal is modulated to the beat frequency f=F.sub.L -F.sub.S.
Heterodyne detection has the advantage compared with homodyne detection of eliminating low frequency noise by filtering around frequency f. Homodyne and heterodyne detectors have the disadvantage of only being usable for a small angle observation field typically of approximately 10.sup.-4 radians.
Moreover, it is known, particularly from the article by J. P. Huignard and A. Marrakchi in "Optics Communications", vol. 38, 1981, p. 249 to amplify a wave front in photorefractive crystals such as Bi.sub.12 SiO.sub.20 (B.S.O.) by a mixture of two waves.